1. Field of the Invention
The present invention relates to a process in which particles of a particulate material are wetted to be agglomerated or coated. Gas is passed through the particles and the latter are subjected to motion by the gas and/or mechanical means. The particles are wetted by adding a wetting agent to the gas.
2. Description of the Prior Art
In known processes of this kind the particles are introduced into a container together with warm air to generate a whirling layer. A liquid is sprayed into the fluidized material to agglomerate the particles in said particulate material or to coat them, as required. In these processes the particles are heated approximately to the temperature of the warm air, which may have disadvantages when thermolabile substances are being processed. Proteins, for instance, can be denatured. Furthermore, particularly if the batches are large, it is not possible, or only with considerable expenditure of time, to uniformly wet all of the surfaces of all of the particles by means of the sprayed liquid, as is required for forming homogeneous agglomerates or coatings.
It is known from the International Disclosure Publication WO 82/03972, as well as the corresponding U.S. Pat. No. 4,543,906, and the U.S. Pat. No. 4,476,804 to pass warm air through a bed of particles disposed within a rotating drum and to spray a liquid material onto the particles. The problems that arise in this process are similar to those mentioned before in conjunction with the agglomeration or coating in a whirling layer.
An agglomeration process is known from the German Disclosure Publication 32 04 466, in which jets of overheated steam are introduced by means of steam nozzles into a whirling layer consisting of warm air and a particulate material. In this process, the steam jets are each surrounded by a jet of warm air introduced coaxially thereto. The Disclosure Publication specifies the various steam and air jets to first mix with each other, to make the steam cool off. Then the steam should condense on the particles.
However, in actual reality the process cannot very well proceed in this manner. Since the exit openings of the steam nozzles are located in the region of the whirling bed, the possibility exists, that particles may come in contact with the hot steam, before the steam had a change to cool off, so that if the particles consist of a thermolabile material, they may suffer damage. Since the steam is intended to cool off by becoming mixed with warm air, it seems rather difficult to dose the steam supply in a way to make the steam condense to a large extent and form a mist during the time it gets mixed with the warm air. The formation of a mist contributes to the phenomenon, that a lot of water is carried away with the air sucked upwardly, so that the filters disposed above the whirling bed may become wet. More important is, however, that the particles present in the whirling bed will take on approximately the temperature of the warm air relatively fast. The particles are in addition heated by the condensing heat of the vapor. At that moment, however, the particles are at most slightly less cool than the part of the supplied steam cooled off to the temperature of the warm air. Accordingly, the quantities of steam condensing on the particles are negligible. Furtheron, the particulate materials are also heated in this process so that materials sensitive to high temperatures may also be damaged.